Moved Downloader to http folder #15
Ref. #18
Changes `require` to `require_once`; Refs #6
Refs #14;

*   Renames `model.php` to `adapter/base.php`
*   Renames `db.php` to `dbmanager.php`
*   Changes `require` to `require_once`
Updates documentation for adapters; Refs #14
Throws library specific exception now; Closes #14
modified test files to adopt recent namechanges of files